One of the most important uses of fluorinated elastomers is the manufacture of seals for both static and dynamic utilizations. A particularly important field in this respect is that of the sealing on rotary shafts by means of oil seal rings. In such case it is absolutely necessary that a perfect adhesion capable of remaining quite unaltered even at high temperatures (200.degree.-250.degree. C.) may take place between the fluoroelastomer constituting the sealing element and the metal insert.
In the known processes for vulcanizing fluoroelastomers, diamine compounds such as carbamates were often used in the past and, to a lesser extent, also at present. However, these accelerating-vulcanizing agents though permitting to obtain satisfactory adhesion values, exhibit the drawback of rendering the processing little safe due to the occurrence of scorching phenomena.
With the systems based on bisphenol cross-linking agents, such as for example bisphenol AF, on accelerators based on quaternary salts of phosphonium or of amino-phosphonium, in which the anion is mostly chloride, bromide or tetrafluoroborate, scorching phenomena are avoided, but the adhesion values are low. When, by means of proper formulations, nearly acceptable adhesion values are reached, these are limited to a narrow concentration range of the accelerating agent, as is proved by examples 34 through 36 and 46 through 48.